Electrochemical and photoelectrochemical reduction of furfurals

ABSTRACT

Electrochemical cells and photoelectrochemical cells for the reduction of furfurals are provided. Also provided are methods of using the cells to carry out the reduction reactions. Using the cells and methods, furfurals can be converted into furan alcohols or linear ketones.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under DE-SC0008707awarded by the US Department of Energy. The government has certainrights in the invention.

BACKGROUND

Biomass conversion can provide a viable pathway to minimize ourdependence on petroleum products for generating both fuels and organicchemicals used in various industrial processes. In particular, theconversion of cellulosic matter, the most abundant organic material onearth, is of great interest because it can provide numerous chemicalsand materials. 5-hydroxymethylfurfural (HMF) is one of the mostimportant intermediates of biomass conversion and is obtained by thedehydration of hexoses, most predominantly fructose and glucose. HMF canundergo oxidation, reduction, and etherification to produce a variety ofimportant chemicals. Of the many HMF reduction products,2,5-bis(hydroxymethyl)furan (BHMF), which is produced by reduction ofthe formyl group of HMF, is an important starting molecule for variousindustrially important polymerization or etherification processes (e.g.those use in the production of biodiesel, polyester, and polyurethanefoams).

To date, the conversion of HMF to BHMF has been mainly achieved by usingH₂ as the hydrogen source in a pressurized cell (28-350 bars) operatingat high temperatures (403-423 K) using various heterogeneous catalystssuch as PtO, Pt/C, Ru/CeO_(x), Ru/C, and 2CuO.Cr₂O₃ and Au/Al₂O₃.Considering that H₂ itself is a valuable energy source that must beproduced from other primary sources with an energy input, enablinghydrogenation without consuming H₂ would provide an exciting,alternative pathway for reductive biomass conversion.

Recently, the possibility of electrochemically reducing HMF to BHMF andother reduced species utilizing H⁺ or H₂O as the hydrogen source wasdemonstrated. (See Y. Kwon, E. de Jong, S. Raoufmoghaddam, M. T. M.Koper, Electrocatalytic hydrogenation of 5-hydroxymethylfurfural in theabsence and presence of glucose. Chem Sus Chem 6, 1659-1667 (2013); andP. Nilges, U. Schroder, Electrochemistry for biofuel generation:production of furans by electrocatalytic hydrogenation of furfurals.Energy Environ. Sci. 6, 2925-2931 (2013).) This process is veryappealing because it does not consume H₂ and also because the reactioncan be carried out at an ambient pressure and temperature. However, inorder to make electrochemical biomass conversion a reality, developmentof catalytic electrodes that can simultaneously achieve high efficiency,selectivity, and yield is be critical. In addition, a strategy tominimize the electrical energy input necessary to drive the reductionprocess is required to make the electrochemical conversion truly energyefficient.

SUMMARY

Electrochemical cells and photoelectrochemical cells for the reductionof furfurals are provided. Also provided are methods of using the cellsto carry out electrochemical and photoelectrochemical reductionreactions. Using the cells and methods, furfurals can be converted intofuran alcohols or linear ketones.

One embodiment of a method for the electrochemical reduction of afurfural to a furan alcohol is carried out in an electrochemical cellcomprising: an anode in an anode electrolyte solution; and a cathode ina cathode electrolyte solution, wherein the cathode electrolyte solutioncomprises a furfural and the cathode is in electrical communication withthe anode and comprises a material that is catalytically active for thereduction of the furfural to the furan alcohol. The method comprises:creating a potential difference between the anode and the cathode toprovide a flow of electrons from the anode to the cathode, wherein theelectrons at the cathode undergo reduction reactions with the furfuralto form the furan alcohol at a yield of at least 90%. In someembodiments of the methods and cells, the cathode comprises silverhaving a dendritic fractal morphology.

One embodiment of a method for the photoelectrochemical reduction of afurfural to a furan alcohol is carried out in a photoelectrochemicalcell comprising: an anode in an anode electrolyte solution; and acathode in a cathode electrolyte solution comprising a furfural, whereinthe anode and cathode are in electrical communication and at least oneof the anode and the cathode is a photoelectrode comprising asemiconductor. The method comprises: exposing the at least onephotoelectrode to radiation that is absorbed to produce electron-holepairs, wherein electrons are transported to the electrolyte-cathodeinterface where they undergo reduction reactions with the furfural toform the furan alcohol and holes are transported to theelectrolyte-anode interface where they induce an oxidation reaction.

One embodiment of a method for the conversion of furfurals is carriedout in an electrochemical cell comprising: an anode in an anodeelectrolyte solution; and a cathode comprising zinc in an acidic cathodeelectrolyte solution comprising a furfural; wherein the cathode is inelectrical communication with the anode. The method comprises: creatinga potential difference between the anode and the cathode to provide aflow of electrons from the anode to the cathode, wherein the electronsat the cathode undergo reduction reactions with the furfural to form alinear ketone.

One embodiment of a method for the photoelectrochemical reduction of afurfural to a linear ketone is carried out in a photoelectrochemicalcell comprising: an anode in an anode electrolyte solution; and acathode comprising zinc in an acidic cathode electrolyte solutioncomprising a furfural, wherein the anode and the cathode are inelectrical communication and at least one of the anode and the cathodeis a photoelectrode comprising a semiconductor. The method comprises:exposing the at least one photoelectrode to radiation that is absorbedto produce electron-hole pairs, wherein electrons are transported to theelectrolyte-cathode interface where they undergo reduction reactionswith the furfural to form the linear ketone and holes are transported tothe electrolyte-anode interface where they induce an oxidation reaction.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings.

FIG. 1(A). SEM image of Ag_(gd) electrodes.

FIG. 1(B). SEM image of Ag_(sp) electrodes.

FIG. 2(A). Linear Sweep Voltammetry curves (LSVs) of Cu, Ag_(sp), andAg_(gd) electrodes in 0.5 M borate buffer (pH 9.2) without 0.02 M HMF(scan rate: 5 mV/s).

FIG. 2(B) LSVs of Cu, Ag_(sp), and Ag_(gd) electrodes in 0.5 M boratebuffer (pH 9.2) with 0.02 M HMF (scan rate: 5 mV/s).

FIG. 3. Possible hydrogenation mechanisms for HMF. Solid arrowsrepresent hydrogenation pathways that do not involve the formation ofadsorbed H atom (H_(ads)). Dashed arrows indicate additional pathwaysavailable when the formation of H_(ads) is enabled.

FIG. 4(A). Schematic diagram showing the external bias necessary toachieve HMF reduction in a photoelectrochemical cell.

FIG. 4(B). Schematic diagram showing the external bias necessary toachieve HMF reduction in a electrochemical cell.

FIG. 4(C). LSVs obtained from a photoelectrochemical cell and anelectrochemical cell for HMF reduction using a two-electrode setup in0.5 M borate buffer (pH 9.2) containing 0.02 M HMF (scan rate: 5 mV/s,illumination for photoelectrochemical cell: AM 1.5 G, 100 mW/cm²).

FIG. 5. The reduction schemes for the hydrogenation of HMF,5-methylfurfural and furfural using H⁺ from H₂O as the hydrogen source

FIG. 6. LSVs of Zn electrodes in 0.2 M sulfate buffer (pH 2) without andwith 0.02 M HMF. Scan rate: 5 mV/s, 3 cm² electrodes.

FIG. 7. The reduction schemes for the reductive ring opening of HMF,5-methylfurfural and furfural.

DETAILED DESCRIPTION

Electrochemical cells (ECs) and photoelectrochemical cells (PECs) forthe reduction of furfurals are provided. Also provided are methods ofusing the cells to carry out electrochemical and photoelectrochemicalreduction reactions. Using the cells and methods, furfurals can beconverted into furan alcohols or linear ketones.

The cells and methods are able to convert HMF, a common biomass-derivedintermediate, into organic building block molecules that are useful inthe production of a variety of chemicals. The reduction products include2,5-hexanedione (HD), which can be converted to 2,5-dimethylfuran (DMF),a highly efficient biofuel. The reductions can be conducted under mildconditions at ambient temperatures and pressures (e.g., about 23° C. andabout 1 atm) using water, an abundant hydrogen source. Even under thesemild conditions, the furfural reduction competes favorably with waterreduction to provide furfural reduction products with high yields andselectivities. This is advantageous because the electrochemical andphotoelectrochemical reduction of furfurals, such as HMF, to furanalcohols, such as BHMF, is thermodynamically more favorable than thereduction of water and because it eliminates inefficiencies related tothe production, storage and use of H₂. In addition, when the reductionreactions are used as the cathode reactions in a PEC the energyefficiency of the cells is enhanced by the use of photogenerated chargecarries.

One aspect of the present technology provides cells and methods for theelectrochemical or photoelectrochemical reduction of furfurals to furanalcohols via hydrogenation. An embodiment of an EC for carrying out theelectrochemical reduction of the furfurals comprises an anode in ananode electrolyte solution; and a cathode in a cathode electrolytesolution. The electrolyte solutions may be aqueous or non-aqueous.However, if a non-aqueous cathode solution is used, a proton donorshould be provided and the electrolyte and solvent should not beelectrochemically active within the range of HMF reduction. The cathodeelectrolyte solution comprises a furfural. The cathode, which is inelectrical communication with the anode, comprises a material that iscatalytically active for the reduction of the furfural to a furanalcohol. The operation of the EC is illustrated in FIG. 4B and describedin detail in the Example 1. A more general description ofelectrochemical reduction of HMF in an aqueous electrolyte solution isprovided here. To operate the EC, one or more voltage sources are usedto create a potential difference between the anode and the cathode, suchthat a flow of electrons from the anode to the cathode through andexternal wire results. The electrons at the surface of the cathode thenundergo reduction reactions with the furfurals in the cathodeelectrolyte solution, while oxidation reactions occur at the anode.

An embodiment of a PEC for carrying out the electrochemical reduction ofthe furfurals comprises an anode in an anode electrolyte solution; and acathode in an aqueous cathode electrolyte solution. At least one of theanode and the cathode is a photoelectrode comprising an n-type or p-typesemiconductor, respectively. The cathode electrolyte solution comprisesan electrolyte and a furfural. As in the EC, the cathode is inelectrical communication with the anode and comprises a material that iscatalytically active for the reduction of the furfural to a furanalcohol. The operation of the PEC is illustrated in FIG. 4A anddescribed in detail in Example 1. In this PEC, the anode is aphotoanode. A more general description of the photoelectrochemicalreduction of furfurals in an aqueous electrolyte solution is providedhere. To operate the PEC, a photoanode comprising a semiconductormaterial is irradiated with electromagnetic radiation to createelectron-hole pairs, which are separated in the photoanode. Theelectrons are then driven through an external wire from the photoanodeto the cathode. A potential may be applied to the photoanode to helpdrive the PEC reactions. The electrons at the surface of the cathodethen undergo reduction reactions with the furfural in the cathodeelectrolyte solution, while oxidation reactions occur between holes atthe surface the photoanode and species contained in the anodeelectrolyte solution. An advantage of the PEC is that, by using solarradiation to drive the oxidation and reduction reactions, the celloperation becomes more efficient from an energy usage standpoint.

In the embodiments of the EC and PEC shown in FIGS. 4A and 4B HMF isreduced to BHMF. BHMF is useful because it can be used as across-linking agent in the production of polymers, such aspolyurethanes. However, the cells can be used to reduce other furfuralsto other furan alcohols, as discussed below. The anode reactionillustrated in FIGS. 4A and 4B is the oxidation of water to oxygen.However, other anode reactions can be employed.

The photoanode in the PEC may be composed of a variety ofsemiconductors. In some embodiments of the PECs, the photoanodecomprises n-type BiVO₄, desirably nanoporous BiVO₄ with a coating of oneor more oxygen evolution catalysts. Methods of fabricating an n-typeBiVO₄ photoanode are described in Kim, T. W. & Choi, K.-S. NanoporousBiVO₄ Photoanodes with Dual-Layer Oxygen Evolution Catalysts for SolarWater Splitting. Science 343, 990-994 (2014). Although the cathode inthe PEC of FIG. 4A is not a photocathode, a photocathode comprising ap-type semiconductor can be used instead of, or in addition to, aphotoanode.

Cathode materials that are catalytically active for the reduction offurfurals to furan alcohols include silver and indium. An example of acatalytically active cathode material that works well is silver having adendritic fractal morphology, as shown in FIG. 1A. The catalyticallyactive material may be supported or unsupported. For example, thecathode may comprise a film or particles of the catalytically activematerial on a non-catalytic substrate. If the reduction is conducted ina PEC, the cathode may be a photocathode comprising a substratecomprising a p-type semiconductor, the surface of which is partiallycoated with the catalytically active material. In a PEC that includes aphotocathode, photogenerated electron-hole pairs are created and thenseparated in the p-type semiconductors and the electrons are transportedto the catalytically active material where they are consumed in thereduction reactions.

In addition to HMF, the furfurals that can be electrochemically orphotoelectrochemically reduced include furfural itself(furan-2-carbaldehyde), which can be reduced to 2-furanmethanol. Thefurfurals further include furfurals with different ring substituents,such as hydroxyl or alkyl substituents. These include 5-methylfurfural(5-MF), which can be reduced to 5-methylfurfuryl alcohol (5-MFA). 5-MFAis an intermediate in the production of DMF. The reduction schemes forthe hydrogenation of HMF, 5-MF and furfural using H⁺ from H₂O as thehydrogen source are shown in FIG. 5.

The present electrochemical and photoelectrochemical furfural reductionsto furan alcohols can be carried out in cathode electrolyte solutions atrelatively low pH. For example, using the present cells and methods, HMFcan undergo reduction at a pH of 10 or lower. This includes embodimentsof the cells and methods in which HMF is reduced at a pH of 9.5 orlower. The use of lower pH solutions is advantageous because HMF is morestable under these conditions, however, higher pH conditions can beused. For example, the reductions can be carried out in a pH range fromabout 6 to about 11, including in the range from 8 to 10. Higher pHconditions will work, but at higher pH polymerization products maybecome significant. Lower pH conditions will also work, but may generatea smaller yield of the alcohols, with other products (e.g.,ring-opening) and water reduction becoming more dominant. The solutionsmay be buffered to maintain a given pH. The electrolyte solvent for theanode may be the same as or different from that of the cathode and maybe aqueous or non-aqueous.

The electrochemical and photoelectrochemical reduction of furfurals canbe carried out substantially completely to provide products at a highyield with high selectivity. For example, the furfurals can be convertedinto furan alcohols with a furan conversion of at least 80%, a productyield of at least 80% and/or a product selectivity of at least 80%. Thisincludes embodiments of the cells and methods that provide furanconversions, furan alcohol yields and/or furan alcohol productselectivities of at least 85%, at least 90%, at least 95% and at least99%. Methods for quantifying conversions and yields are described in theexamples that follow.

Another aspect of the present technology provides cells and methods forthe electrochemical or photoelectrochemical reduction of furfurals tolinear ketones via reductive ring opening, which may occur through apseudo-Clemmensen reduction. The electrochemical andphotoelectrochemical reduction of furfurals to form linear ketones canbe carried out in ECs and PECs having the same construction and generaloperating principles as those shown in FIGS. 4A and 4B. However, thecathode in the cells for linear ketone production comprise zinc as acatalyst for the furfural ring opening reactions in place of a materialthat is catalytically active for the reduction of the furfural to thefuran alcohol. Also, in the cells for linear ketone production, thecathode electrolyte solution is acidic. As in the case of furfuralreduction to furan alcohols, a variety of anode reactions, including theoxidation of water to oxygen, can be employed.

In order to promote the formation of the linear ketones, the electrolytesolution desirably has a pH of no greater that about 5, more desirablyno greater than about 4 and still more desirably no greater than about2.5. In some embodiments of the ECs and PECs, the cathode electrolytesolution has a pH of about 2. Lower pH conditions can be used, however,if the pH is too low the acidic conditions may lead to HMFpolymerization. The cathode electrolyte solution may comprise a pHbuffer in order to maintain a stable pH throughout the reduction.

Furfurals that can be converted into linear ketones include, but are notlimited to, HMF, 5-MF and furan-2-carbaldehyde (furfural). HMF and 5-MFare reduced to HD, while furan-2-carbaldehyde is reduced to4-oxopentanal. The 4-oxopentanal can then be further reduced to5-hydroxy-2-pentanone. The reduction schemes for HMF, 5-MF and furfuralusing H⁺ from H₂O as the hydrogen source are shown in FIG. 7.

As shown in the figure, the reduction product HD can subsequently beconverted into DMF via dehydration. Thus, the present cells and methodsprovide a critical step in a pathway to DMF production.

The electrochemical and photoelectrochemical reduction of furfurals canbe carried out substantially completely to provide products at a highyield with high selectivity. For example, the furfurals can be convertedinto linear ketones with a product yield of at least 70%. This includesembodiments of the cells and methods that provide a linear ketone yieldof at least 75%, at least 80% and at least 85%.

Example 1

This example demonstrates the reduction of HMF to BHMF with Ag catalyticelectrodes, which achieved Faradaic efficiency (FE) and yield nearing100%, using H₂O as the hydrogen source under ambient pressure andtemperature. It further demonstrates the construction of aphotoelectrochemical cell where solar energy can be directly utilizedfor HMF reduction to BHMF, decreasing the external energy inputnecessary for reduction. Previously, for cleaner and more sustainablebiomass conversion, it was envisioned that the H₂ gas required forreductive biomass conversion would be provided from H₂ produced byrenewable sources such as solar water splitting. However, an even moreattractive and energy efficient route would be the direct utilization ofsolar energy and water for HMF reduction without involving the formationof H₂. The photoelectrochemical cell presented in this exampledemonstrates the feasibility of this approach, which can eliminatemultiple issues dealing with production, storage, and use of H₂, all ofwhich require significantly more energy and costs.

When HMF is reduced in an aqueous solution, water reduction to H₂ is themajor competing reaction. Therefore, in order to increase the FE of HMFreduction, electrodes that are poorly catalytic for the H₂ evolutionreaction, such as silver and copper, were investigated, which revealedthat Ag electrodes are the more effective for BHMF formation. Highsurface area Ag electrodes used in this study were prepared bygalvanically displacing a piece of commercially available Cu foil withAg in a 0.05 M AgNO₃ solution for 30 s. The resulting Ag electrodeobtained is referred to as Ag_(gd) for the rest of this example. Thedisplacement reaction was kinetically fast and quickly depleted Ag⁺ ionsat the Cu surface. As a result, the growth of Ag was limited bydiffusion of Ag⁺ ions, and Ag electrodes with a dendritic fractalmorphology were obtained (FIG. 1A). This morphology is favorable forincreasing surface area while maintaining good electrical continuitybetween Ag crystals.

For comparison, Ag films with flat and smoother surfaces were alsoprepared by sputter coating Ag onto Cu foil, which will be referred toas Ag_(sp) (FIG. 1B). The X-ray diffraction (XRD) patterns of Ag_(gd)and Ag_(sp) electrodes were obtained. Both films were composed ofcrystalline Ag and no other crystalline impurity peaks (e.g. Ag₂O, AgO)were observed.

The electrocatalytic properties of Ag electrodes for HMF reduction werefirst examined by performing linear sweep voltammetry (LSV) in anundivided cell containing a 0.5 M borate buffer solution (i.e. 0.5 Mboric acid adjusted to pH 9.2 with sodium hydroxide) with and without0.02 M HMF (FIG. 2). The LSV of the Cu foil used as the substrate forpreparing Ag electrodes was also performed for comparison. The LSVs weremeasured using a three-electrode system composed of a Pt counterelectrode and a Ag/AgCl (4 M KCl) reference electrode. The results wereplotted both against Ag/AgCl and against the reversible hydrogenelectrode (RHE). The latter allows easy interpretation of the dataagainst the thermodynamic water reduction potential to H₂ in the pHcondition used in this example.

The cathodic currents observed in the solution without HMF are due tothe reduction of water to H₂ (FIG. 2A), which show that Cu is a poorelectrocatalyst for H₂ production, requiring an overpotential of 0.52 Vto achieve a current density of 1 mA/cm². The results also show thatboth Ag_(gd) and Ag_(sp) electrodes were even poorer catalysts than theCu electrode, requiring even more overpotential for H₂ production. Thepotentials required to generate 1 mA/cm² of current density for waterreduction are −0.60 V and −0.67 V vs. RHE for Ag_(gd) and Ag_(sp)electrodes, respectively. While Ag_(gd) and Ag_(sp) show similarcathodic onset potentials for H₂ evolution, the current increases morerapidly for the Ag_(gd) when the potential sweeps to the negativedirection due to the high surface area of its dendritic morphology.

When 0.02 M HMF was added to the solution, all electrodes showed a shiftof the cathodic current onset potential to the positive direction (FIG.2B). This indicates that HMF reduction is easier than water reduction onthe Ag and Cu electrodes. In particular, while Ag_(gd) and Ag_(sp)showed similar onset potential for water reduction, the onset potentialof Ag_(gd) for HMF reduction was significantly more positive than thatof Ag_(sp). In fact, it was even more positive than that of Cu foil,indicating that Ag_(gd) is particularly catalytic for HMF reduction. Thepotentials required to generate 1 mA/cm² of current density for HMFreduction are −0.33 V, −0.44 V, and −0.48 V vs. RHE, respectively, forAg_(gd), Ag_(sp), and Cu foil.

In order to analyze reduction products and determine conversionefficiencies, HMF reduction was carried out in a divided cell at aconstant potential of −1.3 V vs. Ag/AgCl (equivalent to −0.56 V vs. RHE)by passing 20 C in 14 mL of 0.5 M borate buffer solution containing 0.02M HMF. After reduction, the electrolyte was analyzed by ¹H-NMR toquantify the amounts of BHMF and remaining HMF. Based on these results,FE and yield for BHMF formation, which are summarized in Table 1, werecalculated using the following equations.

$\begin{matrix}{{{FE}(\%)} = {\frac{{mol}\mspace{14mu} {of}\mspace{14mu} {BHMF}}{{Total}\mspace{14mu} {charge}\mspace{14mu} {{passed}/\left( {F \times 2} \right)}} \times 100}} & (1) \\{{{Yield}\mspace{14mu} {of}\mspace{14mu} {{BHMP}(\%)}} = {\frac{{mol}\mspace{14mu} {of}\mspace{14mu} {BHMF}\mspace{14mu} {formed}}{{mol}\mspace{14mu} {of}\mspace{14mu} {HMF}\mspace{14mu} {consumed}} \times 100}} & (2)\end{matrix}$

TABLE 1 Results obtained for electrochemical HMF reduction to BHMF byAg_(gd), Ag_(sp), and Cu electrodes at E = −1.3 V vs. Ag/AgCl (=−0.56 Vvs. RHE) for 20 C passed. Average current HMF BHMF Rate for BHMF Elec-density consumed formed production FE Yield trode (mA/cm²) (μmol) (μmol)(μmol/h · cm²) (%) (%) Ag_(gd) 6.24 103 102 114 98.1 98.6 Ag_(sp) 2.4686.1 74.2 32.8 71.6 86.2 Cu 1.32 52.5 19.9 4.7 19.2 37.9

The FE and yield for BHMF formation by the Ag_(gd) electrode were,remarkably, 98.1% and 98.6%, respectively. This means that formation ofH₂ and other HMF reduction products is negligible. The FE and yield forBHMF formation were 71.6% and 86.2%, respectively, for the Ag_(sp)electrode and 19.2% and 37.9%, respectively, for the Cu foil electrode.The decrease in yield was due to the formation of C—C bonds between HMFmolecules, mainly between a carbonyl carbon of one HMF molecule and acarbon of the furan ring of another HMF molecule, forming dimeric andpolymeric species based on the NMR data. Other commonly expectedhydrogenation or hydrogenolysis products of HMF that do not involveHMF-HMF reactions, such as 5-methylfurfural, 5-methylfurfuryl alcohol,or 2,5-dimethylfuran, were not detected. The FEs of the Ag_(sp) and Cufoil electrodes, which are much lower than their yields for BHMFformation, indicate that part of the cathodic current was used for H₂production.

In addition to FE and yield, the conversion rate of HMF to BHMF per unitgeometric area of electrode, which depends both on the FE and themagnitude of current density, can be another important criterion forevaluating the efficiency of the reduction process; even if the FE ishigh, if the current density itself is low, the conversion rate can beslow. Table 1 shows that Ag_(gd) not only exhibits the highest FE butalso the highest average current density. As a result, the conversionrate by Ag_(gd) is more than 3 times and 24 times faster than those ofAg_(sp) and Cu, respectively.

After electrolysis, the composition, crystallinity, and morphology ofthe Ag_(gd) electrode were reexamined. No sign of instability ordeformation of the Ag surface or composition was observed.

Electrochemical HMF reduction was also performed using the Ag_(gd)electrode at various applied potentials, which can provide mechanisticinsights for potential-dependent HMF reduction. The results aresummarized in Table 2.

TABLE 2 Results obtained for electrochemical HMF reduction to BHMF by aAg_(gd) electrode at various potentials for 20 C passed. Average HMFPotential current con- BHMF Rate for BHMF (V vs. density sumed formedproduction FE Yield Ag/AgCl) (mA/cm²) (μmol) (μmol) (μmol/h · cm²) (%)(%) −1.0 0.414 60.4 27.4 2.04 26.4 45.4 −1.05 1.21 99.6 63.4 13.7 61.163.6 −1.1 2.47 101 81.7 36.3 78.8 81.0 −1.2 2.94 103 88.6 46.8 84.8 85.8−1.3 6.24 103 102 114 98.1 98.6 −1.4 7.06 102 101 128 97.4 98.5 −1.57.97 101 99.1 139 95.6 97.8 −1.6 10.2 96.2 93.6 171 90.3 97.3 −1.7 12.087.6 85.7 185 87.1 97.8 −1.8 17.1 61.4 60.1 185 58.0 98.0

Based on the FEs and yields of BHMF, the reduction potential could bedivided into three regions. The first region is −1.0 V≦E<−1.1 V vs.Ag/AgCl. In this region, the amount of BHMF formed was much less thanthe amount of HMF consumed, meaning that there is a competing reactionconsuming HMF, which lowers both the FE and yield for BHMF formation.The major competing reaction was the reaction between HMF molecules,which is the same competing reaction observed for the Cu and Ag_(sp)electrodes at −1.3 V vs. Ag/AgCl discussed above. The LSVs obtainedwithout HMF (FIG. 2A) show that H₂ evolution on the Ag_(gd) electrodeinitiates at −1.1 V vs. Ag/AgCl. Therefore, in this potential region,the reduction mechanism of HMF most likely does not involve theformation of surface adsorbed hydrogen atoms (H_(ads)), which is thefirst step for H₂ evolution. Then, HMF reduction proceeds either throughe⁻ transfer to an HMF molecule to form an anionic intermediate thatfurther reacts with H⁺ or through simultaneous e⁻ transfer and H⁺transfer to HMF. Plausible mechanisms describing these pathways areshown in FIG. 3 with solid arrows. In this case, the surface of theAg_(gd) electrode is expected to be covered only with HMF or itsreduction intermediates. As a result, reactions that form C—C bondsbetween HMF molecules and/or their reduction intermediates can bepossible competing reactions in this potential region.

The second potential region is −1.1 V≦E<−1.6 V vs. Ag/AgCl where boththe FE and yield for BHMF formation increase considerably and becomenear 100%. This suggests that the reactions between HMF moleculesconsiderably diminish in this potential region. LSVs obtained withoutHMF (FIG. 2A) indicate that H₂ evolution and, therefore, the formationof H_(ads) are possible on the Ag_(gd) electrode surface in this region.This means new mechanisms that involve the formation of H_(ads) for BHMFproduction are possible in this region, which are indicated with dashedarrows in FIG. 3. As the coverage of H_(ads) increases on the electrodesurface and the rate of BHMF formation increases, the probability of HMFmolecules adsorbing in close enough proximity on the electrode surfacedecreases, and the HMF-HMF reactions can be effectively suppressed.

The third potential region is E≧−1.6 V vs. Ag/AgCl, where H₂ evolutioncompetes with BHMF formation, lowering the FE for BHMF formation.However, since there is no competing reaction consuming HMF in thisregion, the amount of HMF consumed is also lowered. As a result, theyield for BHMF remains high in this region. In addition, since thecurrent density in this high overpotential region is significantlyincreased, the conversion rates for BHMF are higher than those in thelow overpotential regions.

The electrochemical reduction of HMF on Ag_(gd) electrodes demonstratedthat electrochemical hydrogenation can be achieved with high FE andyield without using H₂ as the hydrogen source. Therefore, this procedurehas the potential of offering the mildest hydrogenation conditions forHMF. However, electrochemical hydrogenation requires an input ofelectrical energy to drive the reaction. Therefore, in order to make theoverall conversion process more energy efficient, the possibility ofdirectly utilizing solar energy to convert HMF to BHMF was investigatedby constructing a photoelectrochemical cell.

The photoelectrochemical cell used an n-type BiVO₄ electrode as a photonabsorber and as an anode (photoanode) and a Ag_(gd) electrode as thecathode. The nanoporous BiVO₄ electrode used in this example wasprepared using a method reported in a recent paper where it was used asa photoanode for a water splitting photoelectrochemical cell anddemonstrated the highest applied bias photon-to-current efficiency amongall oxide-based photoelectrodes reported to date. (See, T. W. Kim, K.-S.Choi, Nanoporous BiVO₄ photoanodes with dual-layer oxygen evolutioncatalysts for solar water splitting. Science 343, 990-994 (2014).)Because the bare surface of BiVO₄ is poorly catalytic for wateroxidation, the surface of the BiVO₄ electrode was coated with duallayers (FeOOH and NiOOH) of oxygen evolution catalysts (OEC). Thedetailed synthesis methods and the role of dual layer OECs can be foundelsewhere. (See, T. W. Kim, K.-S. Choi, Nanoporous BiVO₄ photoanodeswith dual-layer oxygen evolution catalysts for solar water splitting.Science 343, 990-994 (2014).)

Under illumination, electron-hole pairs were generated and separated inBiVO₄. The holes were used at the BiVO₄ surface to oxidize water to O₂,while the photoexcited electrons in the conduction band (CB) of BiVO₄were transferred to the Ag_(gd) cathode to reduce HMF to BHMF (FIG. 4A).The cell reactions are summarized below. The standard free energy change(ΔG°) of the overall reaction at 298 K is calculated to be 211.20kJ/mol. (See, Y. Park, K. J. McDonald, K.-S. Choi, Progress in bismuthvanadate photoanodes for use in solar water oxidation. Chem. Soc. Rev.42, 2321-2337 (2013).)

HMF+2H⁺+2e ⁻→BHMF  Cathode Reaction:

H₂O→½O₂+2H⁺+2e ⁻  Anode Reaction:

HMF+H₂O→BHMF+½O₂  Overall:

Since the photoexcited electrons in the CB of BiVO₄ already possess asignificantly negative potential (˜0.0 vs. RHE), the additionalpotential necessary to reduce HMF to BHMF can be decreased considerably.Also, since the valence band (VB) edge of BiVO₄ is located at ˜2.4 V vs.RHE, the photogenerated holes in the VB of BiVO₄ have sufficientoverpotential for water oxidation before applying any external bias,which decreases the external bias necessary for the cell operation. Thisis quite different from the case of an electrochemical cell wheresufficient overpotentials for both the cathode reaction (HMF reduction)and for the anode reaction (water oxidation) need to be provided by theexternal bias (FIG. 4B).

The advantage of using solar energy in decreasing external bias toreduce HMF is well demonstrated in FIG. 4C comparing an LSV obtainedfrom an electrochemical cell composed of a Ag_(gd) cathode and a Ptanode with that obtained from a photoelectrochemical cell composed of aAg_(gd) cathode and a BiVO₄ photoanode under AM 1.5 G illumination. Theelectrolyte was a 0.5 M borate buffer solution (pH 9.2) containing 0.02M HMF. In order to achieve a current density of 1 mA/cm² in anelectrochemical cell, application of 2.76 V between a Ag_(gd) cathodeand a Pt anode was necessary. However, when a photoelectrochemical cellis used, 1 mA/cm² was achieved at a potential of 0.92 V between theBiVO₄ photoanode and Ag_(gd) cathode, saving about 2 V.

It should be noted that the LSVs shown in FIG. 4C were obtained using atwo-electrode cell. This is critical for properly evaluating potentialinput necessary for the cell operation. LSVs obtained using a threeelectrode cell such as those shown in FIG. 2 display the change incurrent density when the potential between the working electrode (WE)and the reference electrode (RE) is varied. However, this experimentdoes not provide any information on the potential applied between the WEand the counter electrode (CE). Therefore, although the data obtainedfrom a three-electrode cell is important for analyzing the reactionoccurring at the WE at a precisely controlled potential against the RE,it cannot be used for evaluating the energy input or efficiency of thecell.

In order to analyze products, FE, and yield, photoelectrochemicalreduction of HMF was carried out at a constant potential of 1.5 Vbetween the BiVO₄ photoanode and the Ag_(gd) cathode. According to theLSVs shown in FIG. 4C, electrochemical reduction of HMF at 1.5 V shouldbe negligible when a two electrode cell is used. The results show thatBHMF was produced with a FE of 94.1% and a yield of 94.8% (Table 3),confirming that the results obtained from electrochemical HMF reductioncan be reproduced by a photoelectrochemical cell while decreasing thenecessary electrical energy input. When the cell design is improved tominimize the potential loss in the solution and through the frit, and tofacilitate the mass transport, external electrical energy input will befurther reduced.

TABLE 3 Faradaic efficiency, selectivity, and conversion rate for BHMFformation from photoelectrochemical HMF reduction. A Ag_(gd) workingelectrode and a BiVO₄ counter electrode (photoanode) were used at 1.0 Vunder illumination (AM 1.5G, 100 mW/cm²) for 5 C passed. Average HMFcurrent con- BHMF Rate for BHMF Potential density sumed formedproduction FE Yield (V) (mA/cm²) (μmol) (μmol) (μmol/h · cm²) (%) (%)1.0 1.12 25.7 24.4 19.6 94.1 94.8

The successful photoelectrochemical reduction of HMF suggests numerouspossibilities for performing reductive biomass conversion using solarenergy and water as the hydrogen source under ambient conditions. Whilewater reduction and CO₂ reduction have been mainly investigated ascathode reactions of photoelectrochemical cells to date, employingreductive biomass conversion reactions as cathode reactions ofphotoelectrochemical cells will provide more freedom in the constructionof photoelectrochemical cells, which will increase the overallconversion efficiency while resolving practical issues such ascollection and separation of reaction products.

Detailed Materials and Methods

1. Chemicals and Materials

Copper foil was purchased from Nimrod Hall Copper Foil Company. Boricacid (≧98.5%), sodium hydroxide (≧97%), and 5-hydroxymethylfurfural(≧99%) were purchased from Sigma Aldrich. Silver nitrate (99.9+%) waspurchased from Alfa Aesar. To assist in method development, standardcalibration, and product identification, 2,5-bis(hydroxymethyl)furan(97%) was purchased from Polysciences, Inc. All chemicals were usedwithout further purification.

2. Preparation of Electrodes

Cu electrodes used in this study were prepared by cutting Cu foil topieces with a dimension of 1.5 cm×2.5 cm. Cu tape was attached to thetop of the Cu foil electrode to enable connection to the lead to thepotentiostat. The backside and top 0.5 cm of the Cu foil were thencovered with Teflon tape to yield a 3.0 cm² working area. Ag_(gd) filmswere prepared by immersing clean Cu foil electrodes (1.5 cm×2.0 cmworking area) into a 50.0 mM solution of AgNO₃ for 30 seconds. Theconcentration of the Ag⁺ solution and the duration of depositionreported here are optimum conditions to produce uniform and adherent Agelectrodes. Increasing the Ag⁺ concentration, which increased thedeposition rate of Ag, resulted in poor adhesion of Ag on the Cusurface, while decreasing the Ag⁺ concentration resulted in nonuniformcoverage of Ag on the Cu surface. In a similar manner, increasing ordecreasing the deposition time affected the adhesion or uniformity ofthe Ag deposition. To achieve good coverage and adhesion of Ag on theCu, the Cu foil electrodes were cleaned just prior to deposition byrinsing with 2-propanol and water followed by immersing in 1 M HCl for 1minute to remove surface oxides. They were then rinsed with water,dried, and immediately placed in the AgNO₃ solution. Followingdeposition, the film was rinsed gently with water and dried in an airstream. To prepare Ag_(sp) electrodes, Cu foil substrates cleaned usingaforementioned procedures were placed in a sputter coater (Anatech USA,DC/RF Dual Source Sputtering System) where 100 nm of Ag wassputter-coated onto the Cu substrate. The films were then made intoelectrodes in the same manner as for the Cu foil electrodes. The Ptcounter electrodes were prepared by sputter coating a 100 nm platinumlayer over a 20 nm titanium layer onto cleaned glass slides. Thedimension of the working area of the Pt counter electrode was 2.0 cm×2.0cm.

The nanoporous BiVO₄ electrode used in this study was prepared using amethod reported in a recent paper where it was used as a photoanode fora water splitting photoelectrochemical cell. Because the bare surface ofBiVO₄ is poorly catalytic for water oxidation, the surface of the BiVO₄electrode was coated with dual layers (FeOOH and NiOOH) of oxygenevolution catalysts (OEC). The synthesis methods and the role of duallayer OECs can be found elsewhere.

3. Characterization

Ag_(gd) and Ag_(sp) electrodes were characterized by powder X-raydiffraction (XRD) (Bruker D8 Advanced PXRD, 298 K, Ni-filtered CuK_(α)-radiation, λ=1.5418 Å). Aside from the underlying coppersubstrate, only crystalline Ag peaks were observed for both films. WhenXRD patterns of the electrodes were obtained again after passing 20 Cfor HMF reduction at −1.3 V vs. Ag/AgCl, no change was observed. Thesurface morphologies of the Ag_(gd) and Ag_(sp) electrodes were examinedby Scanning Electron Microscopy (SEM) using a LEO 1530 microscope at anaccelerating voltage of 5 kV before and after electrochemical HMFreduction at −1.3 V vs. Ag/AgCl. Again, no change was observed.

4. Electrochemical HMF Reduction

To investigate the catalytic abilities of Cu foil, Ag_(gd), and Ag_(sp)electrodes to reduce HMF, linear sweep voltammetry (LSV) was performedusing solutions with and without HMF. The general experimental setupconsisted of a three-electrode system (Cu or Ag working electrode, Ptcounter electrode, and a Ag/AgCl (4 M KCl) reference electrode) in anundivided cell controlled by a Bio-Logic VMP2 potentiostat. A 0.5 Mborate buffer solution (pH 9.2) with and without 0.02 M HMF was used asthe electrolyte, which was purged with N₂ to remove dissolved oxygenprior to obtaining LSVs. The potential was swept to the negativedirection from the open circuit potential to −1.5 V vs. Ag/AgCl at 5mV/s without stirring. All current densities reported in this examplewere calculated based on the geometric area of the working electrode.

Constant potential electrolysis experiments were conducted to quantifyproducts, yields, and Faradaic efficiency. A three-electrode setup wasemployed using an H-shaped cell divided by a glass frit. The Cu or Agworking electrode and the reference electrode were immersed in thecathodic compartment containing 14 mL of a 0.5 M borate buffer solution(pH 9.2) with 0.02 M HMF. The counter electrode was immersed in theanodic compartment containing 14 mL of a 0.5 M borate buffer solution(pH 9.2) with 1 M sodium sulfite. Because sulfite oxidation occurs at amore positive potential than water oxidation, using sulfite oxidation asthe counter electrode reaction prevented the drift of the anodepotential to the potential limit of the potentiostat, which occasionallyoccurred when water oxidation was used as the counter electrode reactionbecause the divided cell used in this study was not optimized and causeda significant IR drop during electrolysis. For a three-electrode setup,the counter electrode reaction does not affect the current generated atthe working electrode. Therefore, using sulfite oxidation as the counterelectrode reaction allowed us to prevent experimental issues unrelatedto HMF reduction without affecting the results obtained at the workingelectrode.

Just prior to use in electrolysis, the working electrode was reduced ina separate borate buffer solution by performing a potential sweep fromopen circuit potential to −1.5 V vs. Ag/AgCl at 5 mV/s to ensurereduction of any surface oxide species so as to minimize Faradaic lossduring HMF reduction. The designated potential was applied to theworking electrode by a Bio-Logic VMP2 potentiostat for a duration of 20C, after which time the solution was collected and analyzed. Masstransport of the HMF solution during the electrolysis was facilitated bymagnetic stirrer at approximately 1600 rpm.

5. Product Analysis

Products were detected and quantified using a Bruker Avance III 400 MHznuclear magnetic resonance (NMR) spectrometer. Calibration curves weregenerated for both BHMF and HMF by obtaining NMR spectra for a series ofsolutions of known concentration for each species and plotting the areaobtained for signature HMF and BHMF peaks for each concentration. Thearea for HMF and BHMF peaks of unknown concentrations from the productsolution were then plotted against the calibration curves to determinetheir concentrations. The HMF signal in the product solution was alsocompared to that of the initial solution to determine the amount of HMFconsumed. Since the samples were aqueous solutions, the NMR spectra werecollected and processed using a water suppression method to remove the¹H signal from water. As a result, signals for peaks near water (4.7ppm) were significantly decreased. Water suppression did not affectquantifications of HMF and BHMF.

6. Photoelectrochemical HMF Reduction

LSVs for photoelectrochemical HMF reduction were performed in anundivided cell without stirring using the same solution and proceduresdescribed above. The only difference was that the Pt anode was replacedby a BiVO₄ photoanode, and the reference electrode was removed to form atwo-electrode system. The main purpose of using a BiVO₄ photoanode is todecrease the electrical energy input (i.e. applied bias between theworking and the counter electrode) necessary for HMF reduction.Therefore, a two-electrode system must be used to evaluate the potentialapplied between the working and counter electrode. Illumination of theBiVO₄ photoanode was achieved through the FTO substrate (back-sideillumination) with an Oriel LCS-100 solar simulator with the intensityof the incident light calibrated to be AM1.5 G, 100 mW/cm². The resultwas compared with LSVs obtained using a two-electrode system composed ofa Ag_(gd) electrode and a Pt counter electrode. For this comparison, thesizes of all working and counter electrodes were kept as 1 cm².

Constant potential electrolysis was performed by applying 1.5 V betweenthe Ag_(gd) cathode and the BiVO₄ photoanode and passing 5 C underillumination (AM 1.5 G, 100 mW/cm²) using a divided cell. A 0.5 M boratebuffer solution (pH 9.2) was used as the electrolyte and the solution inthe cathodic compartment contained 0.02 M HMF. This time sulfite was notadded to the anodic compartment because the potential necessary for fullcell operation with the presence of only HMF and water aselectrochemically active species needed to be evaluated accurately. Thecell reactions are summarized below.

HMF+2H⁺+2e ⁻→BHMF  Cathode Reaction:

H₂O→½O₂+2H⁺+2e ⁻  Anode Reaction:

HMF+H₂O→BHMF+½O₂  Overall:

When a two-electrode system is used, while the potential differencebetween the cathode and the anode is fixed at 1.0 V, the actualpotentials applied to the WE and CE are not controlled and can beinfluenced by various conditions. The electrochemical study on HMFreduction showed that −1.3 V vs. Ag/AgCl (equivalent to −0.56 V vs. RHE)is an optimum potential for the Ag_(gd) electrode to reduce HMF to BHMFwith the highest yield and FE. In order to ensure that the potentialapplied to the Ag_(gd) electrode is near this value, optimization wasperformed by monitoring the potential applied to the Ag_(gd) and theBiVO₄ electrode by measuring the potential between each of theseelectrodes and a Ag/AgCl (4M KCl) reference electrode that was notconnected to a potentiostat using a multimeter. The optimum potentialswere achieved when the size of the BiVO₄ (2 cm²) was 4 times larger thanthe size of the Ag_(gd) electrode (0.5 cm²). Additionally, stirring wasconducted only in the BiVO₄ anode compartment. This facilitated thecurrent generation at the anode, which in turn resulted in theapplication of a more negative potential to the Ag_(gd) cathode togenerate a matching current. As a result, a desired potential, ˜−1.3 Vvs. Ag/AgCl, was achieved at the Ag_(gd) electrode, which resulted inthe production of BHMF with a FE of 94.1% and a yield of 94.8%.

Example 2

This example demonstrates the electrochemical reductive ring opening of5-hydroxymethylfurfural to 2,5-hexanedione. To investigate the catalyticabilities of Zn electrodes to electrochemically reduce5-hydroxymethylfurfural (HMF), linear sweep voltammetry (LSV) wasperformed using solutions with and without HMF. The general experimentalsetup included a three-electrode system—a Zn working electrode (WE), Ptcounter electrode (CE), and a Ag/AgCl (4 M KCl) reference electrode(RE)—in an undivided cell controlled by a Bio-Logic VMP2 potentiostat.0.2 M potassium sulfate buffer solutions (pH 2) with and without 0.02 MHMF were used for the analysis. The solutions were purged with N₂ toremove dissolved oxygen prior to obtaining LSVs, for which the potentialwas swept to the negative direction from the open circuit potential to−1.8 V vs. Ag/AgCl (4 M KCl) at 5 mV/s without stirring. Multiple scanswere performed for each solution to ensure reproducibility of results.The LSVs (FIG. 6) indicate that HMF reduction occurs prior to waterreduction. The current remains quite flat in the region −1.1 V≧E≧−1.5 Vvs. Ag/AgCl (−0.49 V≧E≧−1.19 V vs. RHE) for HMF reduction as opposed tothe current increase observed for water reduction in the solutionwithout HMF, indicating a suppression of water reduction by the presenceof HMF. This is consistent with the visible lack of significant H₂ gasevolution at the surface of the Zn electrode.

Constant potential electrolysis experiments were conducted to quantifyproducts, yields, and Faradaic efficiencies for HMF conversion, and toobserve any potential-dependent product formation. Using athree-electrode setup in an H-shaped cell divided by a glass frit, theZn WE and the RE were immersed in 14 mL of a 0.2 M potassium sulfate orpotassium phosphate buffer solution (pH 2) containing 0.02 M HMF. The CEwas immersed in 14 mL of a 0.2 M sulfate or phosphate buffer solution(pH 2). The designated potential, −1.1 V≧E≧−1.4 V vs. Ag/AgCl (−0.49V≧E≧−1.09 V vs. RHE), was applied to the WE by the potentiostat for aset duration of charge passed (usually 20 C), after which time thesolution was collected and analyzed. Mass transport of the HMF solutionduring the electrolysis was facilitated by magnetic stirrer atapproximately 1600 rpm. Products were detected and quantified using aBruker Avance III 400 MHz nuclear magnetic resonance (NMR) spectrometer,as well as a Shimadzu QP2010-Ultra gas chromatography-mass spectrometer(GCMS).

The major HMF reduction product observed in the stated potential regionwas 2,5-hexanedione (HD) (FIG. 7). Neither intermediate products noreasier 2-electron reduction reactions such as reduction of the formylgroup to BHMF or reduction of the furan ring (without ring opening) wereobserved. The conversion to HD largely demonstrates apotential-independent trend for Faradaic efficiency (FE). The FE for HDformation was calculated using the following equation.

${{FE}(\%)} = {\frac{{mol}\mspace{14mu} {of}\mspace{14mu} {BHMF}}{{Total}\mspace{14mu} {charge}\mspace{14mu} {{passed}/\left( {F \times 6} \right)}} \times 100\%}$

At pH 2, regardless of the buffer type (either sulfate or phosphatebuffer), the highest average FE of 74.9% for HD formation occurs at −1.1V vs. Ag/AgCl (−0.79 V vs. RHE) whereas the average FE for −1.2 V≧E≧−1.4V vs. Ag/AgCl (−0.49 V≧E≧−1.09 V vs. RHE) is near 70%.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method for the electrochemical reduction of afurfural to a furan alcohol in an electrochemical cell comprising: ananode in an anode electrolyte solution; and a cathode in a cathodeelectrolyte solution, wherein the cathode electrolyte solution comprisesa furfural and the cathode comprises a material that is catalyticallyactive for the reduction of the furfural to the furan alcohol, themethod comprising: creating a potential difference between the anode andthe cathode to provide a flow of electrons from the anode to thecathode, wherein the electrons at the cathode undergo reductionreactions with the furfural to form the furan alcohol at a yield of atleast 80%.
 2. The method of claim 1, wherein the furfural is5-hydroxymethylfurfural and the furan alcohol is2,5-bis(hydroxymethyl)furan.
 3. The method of claim 2, wherein the2,5-bis(hydroxymethyl)furan is formed at a yield of at least 98%.
 4. Themethod of claim 3, wherein the material that is catalytically active forthe reduction of the furfural is silver.
 5. The method of claim 3,wherein the material that is catalytically active for the reduction ofthe furfural is silver and the silver has a dendritic fractalmorphology.
 6. The method of claim 1, wherein the furfural is5-methylfurfural and the furan alcohol is 5-methylfurfuryl alcohol. 7.The method of claim 1, wherein the furfural is furan-2-carbaldehyde andthe furan alcohol is 2-furanmethanol.
 8. The method of claim 1, whereinthe material that is catalytically active for the reduction of thefurfural is silver.
 9. The method of claim 8, wherein the silver has adendritic fractal morphology.
 10. The method of claim 1, wherein thematerial that is catalytically active for the reduction of the furfuralis indium.
 11. The method of claim 1, wherein the cathode electrolytesolution has a pH in the range from about 9 to about 9.5.
 12. A methodfor the photoelectrochemical reduction of a furfural to a furan alcoholin a photoelectrochemical cell comprising: an anode in an anodeelectrolyte solution; and a cathode in a cathode electrolyte solutioncomprising a furfural, wherein at least one of the anode and the cathodeis a photoelectrode comprising a semiconductor, the method comprising:exposing the at least one photoelectrode to radiation that is absorbedto produce electron-hole pairs, wherein electrons are transported to theelectrolyte-cathode interface where they undergo reduction reactionswith the furfural to form the furan alcohol and holes are transported tothe electrolyte-anode interface where they induce an oxidation reaction.13. The method of claim 12, wherein the furan alcohol is formed at ayield of at least 90%.
 14. The method of claim 13, wherein the furfuralis 5-hydroxymethylfurfural and the furan alcohol is2,5-bis(hydroxymethyl)furan.
 15. The method of claim 13, wherein thefurfural is 5-methylfurfural and the furan alcohol is 5-methylfurfurylalcohol.
 16. The method of claim 13, wherein the furfural isfuran-2-carbaldehyde and the furan alcohol is 2-furanmethanol.
 17. Themethod of claim 13, wherein the cathode comprises silver having adendritic fractal morphology.
 18. The method of claim 13, wherein thecathode is a photocathode comprising a p-type semiconductor materialpartially coated with silver.
 19. A method for the conversion offurfurals using an electrochemical cell comprising: an anode in an anodeelectrolyte solution; and a cathode comprising zinc in an acidic cathodeelectrolyte solution comprising a furfural, the method comprising:creating a potential difference between the anode and the cathode toprovide a flow of electrons from the anode to the cathode, wherein theelectrons at the cathode undergo reduction reactions with the furfuralto form a linear ketone.
 20. The method of claim 19, wherein the linearketone is 2,5-hexanedione
 21. The method of claim 19, further comprisingdehydrating the 2,5-hexanedione to form 2,5-dimethylfuran.
 22. Themethod of claim 19, wherein the furfural is 5-hydroxymethylfurfural or5-methylfurfural and the linear ketone is 2,5-hexanedione.
 23. Themethod of claim 19, wherein the furfural is furan-2-carbaldehyde and thelinear ketone is 4-oxopentanal.
 24. The method of claim 23, furthercomprising reducing the 4-oxopentanal to form 5-hydroxy-2-pentanone. 25.A method for the photoelectrochemical reduction of a furfural to alinear ketone in a photoelectrochemical cell comprising: an anode in ananode electrolyte solution; and a cathode comprising zinc in an acidiccathode electrolyte solution comprising a furfural, wherein at least oneof the anode and the cathode is a photoelectrode comprising asemiconductor, the method comprising: exposing the at least onephotoelectrode to radiation that is absorbed to produce electron-holepairs, wherein electrons are transported to the electrolyte-cathodeinterface where they undergo reduction reactions with the furfural toform the linear ketone and holes are transported to theelectrolyte-anode interface where they induce an oxidation reaction.